On Sun, 20 May 2001, David Marjanovic wrote:
> > How is this relevant to paleoecology? Opossums in the wild likely have
> > different enough niche requirements so that they can coexist with
> > racoons. For now, anyway. So?
>
> So they are not outcompeted by placentals.
Here is a study crying out to be done, or has it already? How about an
hypothesis such as: possum radiation occurred more rapidly in the
20th. century possibly due to reduction in racoon numbers at the hands of
man. How about: possum niche overlap with racoons is reduced by resource
partitioning--compare the niches of southern vs. newly immigrating
northern possums. And so on. Instead the resources go into: how many
tsunamis, firestorms, acid rain storms does it take to kill a possum?
> Only placentals have ever got into sea and air (hypotheses have been made
> that because because marsupials need quite well-formed forelimbs at their
> crazily early birth,
Yes, to claw their way to the mammary glands. I remember the feeling
well--in adolescence, however.
> marsupials with highly altered forelimbs may be
> impossible; of course, one can easily say that this is based on negative
> evidence). And land -- I'd say that placentals were just faster to
> diversify.
This is a neat trick in itself. Did you know that chromosome
mutations (inversions, etc) are much more frequent in placentals? I
forget why that would be.
> > Or are you saying placentals are it thanks to a series
> > of unfortunate bolide strikes over the Cenozoic.
>
> This is exaggerating it, but chance does play a role in evolution.
I think its role is exaggerated.
> > After all, our big brains are afforded by our reproductive
> > mode.
>
> I've read the contrary -- our brains are so big as to considerably impede
> birth. In a marsupial the brain could grow much larger in the pouch because
> it needn't pass the pelvis.
Except that head architecture must already be set in order to suckle in
pouch--much like your arm example. As it is, no marsupial approaches the
upper limit of placental brain sizes. I must be careful to add that I am
aware brain capacity is neither a one for one for intelligence, nor the
gold standard for evolutionary success. But, it is what it is, and it
does provide access to different niches.
> Just IMHO your last sentence is on the same
> level -- IMHO background extinction is terribly complex and would need to be
> explained for every single species anew, but mass extinctions are something
> totally different.
And I would agree with this if _everything_ were knocked off. I don't
need a discussion, for example, on ecological interactions of species on
the island of Krakatoa! The island was steralized. But this is so far
from the K/T case. Indeed, differential survival must have explanations
for every single species--not that we'll ever know them--and these
explanations must involve concepts like niche, habitat, resources,
etc.
> Mass extinctions affect the whole globe and practically all habitats
> and clades, and they do not grade into background extinction.
Surely there are different grades of mass extinctions--and surely the low
end mass extinctions (of which the K/T was one) may even grade into
background extinction, especially if a number of extinctions were not a
part of it (pterosaurs, enantiornithines, marsupials). Anyway, the term
"background" extinctions is itself suspect. Extinctions should not go on
at an even pace: new critical adaptations come along and change
things. The concept which is being pushed forward is that extinctions are
either background or massive. Why should this be true? Anyway, the
history of life's diversity is not straight lines interrupted by huge
dips. It is bumpy all the way along with some dips bigger than
others. We have a brain which seeks to pigeon hole. But life is more
complex than we can know.
> I hereby claim that "ecological forces" become practically irrelevant when a
> big meteorite strikes. They may affect survivorship and surely affect why,
> in the case of the K-T, the mammals and birds rather than the crocs or frogs
> took over the world, but not the extinction itself.
This seems contradictory. Ecology is either relevant or irrelevant. If
it affects survivorship it's relevant!
> > They don't specify.
>
> Just as I feared :-( . So they don't say much that's testable.
They're assuming we're up on what condylarths are (a big risk in my case,
I admit).
> > However, there was also an increasing
> > dietary diversity in endemic placentals--and an increase in
> > size! _C. magnus_ is the biggest placental ever seen around those parts.
>
> May explain why they were faster to diversify after the K-T, but not why
> more marsupials than placentals died out at the K-T, if I'm correctly
> informed.
The point is, no one is informed on this: we just don't know, and so we
move to default hypothesis of bolide explanation, just as the ancients
moved to default explanation of the universe being supported by huge
turtles.
> Eldredge and Gould's Punctuated Equilibrium? I'm not suggesting that. BTW,
> this is not as extreme as it sounds, it just consists of the hypothesis that
> speciation is a rather sudden event that involves genetic bottlenecks.
No, I was referring to an article I read that Eldredge was quoted words to
the effect of: "No extinctions occur outside of catastrophies." I believe
I supplied this reference in an archive. The statement is ridiculous, but
it shows how far people can go in pushing their agenda.
> > And then, how
> > do you define "catastrophy"? Foxes released onto goose nesting islands
> > recently caused the near extinction of some species. This was
> > catastrophic for the geese, but a simple predation event for the
> > foxes.
>
> But totally uncomparable to a mass extinction like Eocene-Oligocene, K-T,
> Tr-J, P-Tr...
But still without definition. For example, I get the feeling that even if
it were allowed that pterosaurs, enantis, marsupials were not associated
with any bolide, that the term "mass extinction" would still be applied.
> > There is no fossil record of birds across the boundary.
>
> There is one, at least for footprints. 60 cm above the boundary,
> "transitional shorebirds" were alive and well, just like some meters below
> it.
Interesting. But we already know that neornithines were alive
before and after. What about enanti "shore birds". Or were they already
outcompeted/preyed out of this niche. I mean, enantis were a diverse
group throughout the mesozoic and, if their extinction at the hands of
bolide is true, up to the K/T.
> How do you explain the marine extinctions, BTW?
No idea. But if we're talking about a cessation of photosynthesis--and a
loss of productivity, how did fish survive almost unscathed (right?).
> > Of course there was big-time radiation into empty niches after dinosaurs
> > became extinct. But, in terms of niche diversity and morphology, changes
> > were already underway before.
>
> But different ones than after the boundary, AFAIK...
> You know the SVP meeting abstract from 2000 that states that, based on
> postcranial evidence, zhelestids are not related to (cet)artiodactyls but to
> zalambdalestids and therefore have become extinct at the K-T?
What do you mean? All post K/T mammals had pre-K/T ancestors. I'm just
saying that pre-K/T mammals were experiencing crucial dietary and
morphological rise in diverstity. If this is true, why is it true?
> Ah! (Could you give me the ref? :-9 )
I believe it's: Fox, R.C. and G.P. Youzwyshyn. 1994. New primitive
carnivorans (Mammalia) from the Paleocene of western Canada, and their
bearing on the relationships of the order. J. Vert. Paleont. 14:382-404.
And you had also asked for the second post-K/T dinosaur ref: Bajpai, SD.,
and Prasad, G.V.R. 2000. Cretaceous age for Ir-rich Deccan intertrappean
deposits: paleontological evidence from Anjar, western India. Journal of
the geological society of London. 157:257-260. It supposedly reveals
dino egg-shells in unequivocally post-K/T horizons.